Network storage is a common approach to backing up data, making large amounts of data accessible to multiple users, and other purposes. In a network storage environment, a storage server makes data available to client (host) systems by presenting or exporting to the clients one or more logical containers of data. There are various forms of network storage, including network attached storage (NAS) and storage area network (SAN). In a NAS context, a storage server services file-level requests from clients, whereas in a SAN context a storage server services block-level requests. Some storage servers are capable of servicing both file-level requests and block-level requests.
There are two important trends today in the field of network storage. First, companies today more commonly package their products as virtual storage servers (VSSs) instead of as dedicated (special-purpose) physical storage servers (PSSs). This trend is driven primarily by cost considerations (i.e., it allows lower cost, generic server hardware to be used) and made feasible by the widespread use of system virtualization software in data centers. Second, data centers are increasingly starting to use “shared-nothing” storage environments; that is, in place of dedicated network storage servers, the copious DAS in commodity servers is pooled to provide “virtual” network storage systems. This trend is due to the higher capacities of today's hard drives, as well as a desire not to waste available storage space provided by these commodity servers.
Despite the growing prominence of DAS data centers, a data center operator may at some point find it desirable to transition its operations from a DAS-based VSS configuration (a “VSS-DAS” configuration) to a PSS-based configuration. For example, it may be desirable to upgrade from a relatively inexpensive VSS-DAS system to a more powerful PSS-based system. Or, a data center operator may find it desirable to move from a centralized PSS configuration to a potentially less-expensive VSS-DAS configuration. As another possibility, to facilitate a hardware upgrade of a PSS, it may be desirable to temporarily migrate data from the PSS to another storage server, where at least one of the two storage servers involved in the migration uses DAS (or where the two storage servers otherwise do not share storage with each other). As still other possibilities, it may be desirable to migrate from one VSS to another VSS, or from one PSS to another PSS, where at least one of the two storage servers involved in the migration uses DAS (or where the two storage servers otherwise do not share storage with each other).
This sort of data migration has the potential to be very disruptive to client applications that access data maintained by the storage servers. By “disruptive” what is meant is that it causes apparent downtime of the storage server from the viewpoint of a client application.